Display unit and driving method therefor

ABSTRACT

The invention provides an electrochromic display unit with high-quality display. The display unit comprises a transparent electrode ( 1 ), a display layer ( 2 ) which is formed on the transparent electrode ( 1 ) and which changes a color according to the amount of accumulated electrical charges, and an ion conductive layer ( 3 ) formed on the display layer ( 2 ). A plurality of picture electrodes ( 4 ) are formed on the ion conductive layer ( 3 ) on the side opposite to the display layer ( 2 ). The picture electrodes ( 4 ) are driven independently by, for example, a corresponding thin film transistors ( 6 ). In driving, by applying a drive current having given amount of electrical charges and then applying a certain amount of an inverted current, a certain amount of coloration is deducted, and extra coloration of the display layer ( 2 ) is eliminated.

TECHNICAL FIELD

The present invention relates to a display unit suitable for a displaydevice used for the purpose of reading documents (so-called electronicpaper) and its driving method.

BACKGROUND OF THE INVENTION

In these years, in connection with popularization of the network,documents which were previously distributed in the form of printedmaterials have been delivered in the form of so-called electronicdocuments. Further, books and journals are also increasingly provided inthe form of so-called electronic publishing. A conventional techniqueused to read such information is to read the information from a CRT(cathode ray tube) or a liquid crystal display of the computer. However,it is pointed out that it is impossible to use a luminescent typedisplay such as the CRT for long hours of reading and the like sincesignificant fatigue is incurred based on ergonomic reasons. It is saidthat a non-emissive type display such as the liquid crystal display isnot also suitable for reading due to flicker specific to a fluorescenttube. Further, in either case, there is a problem that a reading placeis limited to where the computer is installed.

In these years, though a reflective liquid crystal display which uses noback light has been practically used, a reflectance in the case of theliquid crystal is in the range from 30 to 40%. These figures meansignificantly bad visibility compared to a reflectance of the printedpapers (reflectance of 75% for OA sheets and paperback books; andreflectance of 52% for newspapers). In addition, since fatigue is easyto be incurred due to dazzle by a light reflector or the like, it isimpossible to use this reflective LCD for long hours of reading.

Therefore, in order to solve these problems, so-called paperlikedisplays or electronic papers have been developed. Display methods usedfor them include an electrophoretic migration method, bicolor balldisplay method, an electrochromic method and the like. In a display withthe electrophoretic migration method (electrophoretic image display:EPID), a white pigment, a black toner and the like are layered on theelectrode by the action of electric field. A display with the bicolorball display method (twisted ball display: TBD) comprises a sphere whosehalf is colored in white and whose another half is colored in black, andutilizes revolutions by the action of electric field. However, since inboth methods, a clearance allowing fluids to gain entry is required andclosest packing is impossible, high contrast is hard to be obtained.Further, there is a problem that the practical writing speed (within 1sec) cannot be obtained unless a drive voltage is 100 V or more.Compared to the displays using these display methods, a display with theelectrochromic method (electrochromic display: ECD) is superior to thedisplays with the foregoing methods in terms of high contrast, andalready used practically as a display for, for example, photochromicglasses and timekeepers.

However, as for the electrochromic display, in the case where charactersand images are displayed by combining fine picture elements with asimple matrix drive method, there is the danger that its display qualityis lowered since its contrast is uniformised due to cross talks betweenpicture elements. Therefore, it is said that an active matrix drivemethod which arranges active devices such as a transistor for everypicture element is desirable. For example, conventionally, anelectrochromic display layer is formed on a glass substrate (TFTsubstrate) wherein thin film transistors (TFT) for every picture elementand wiring electrodes or the like are formed. However, in suchconventional construction, there is a problem that the display qualitiessuch as luminance and contrast are lowered since the electrochromicdisplay layer is observed from the TFT substrate side so that areasoccupied by the TFT and the wiring electrode and the like become shadow.

In light of the foregoing problems, it is an object of the invention toprovide a display unit using the electrochromic method which can providehigh quality displays and its driving method.

SUMMARY OF THE INVENTION

A display unit of the invention comprises a transparent electrode; adisplay layer which is formed on the transparent electrode and change acolor according to the amount of accumulated electrical charges; and anion conductive layer which is formed on this display layer on the sideopposite to the transparent electrode. In the display unit, a pluralityof independent electrodes are formed on the ion conductive layer on theside opposite to the display layer.

A driving method for the display unit of the invention is the drivingmethod for the display unit which comprises the transparent electrode;the display layer which is formed on the transparent electrode andchange a color according to the amount of accumulated electricalcharges; and the ion conductive layer which is formed on this displaylayer on the side opposite to the transparent electrode, wherein aplurality of independent electrodes are formed on the ion conductivelayer on the side opposite to the display layer. In this driving method,the amount of accumulated electric charges of the display layer arecontrolled by selectively supplying a drive current having the amount ofelectric charges according to a coloration density or a coloration areabetween the plurality of independent electrodes and the transparentelectrode, and by controlling the amount of electric charges ordirections of the drive current.

In the display unit according to the invention, characters and imagesdisplayed by the display layer are viewed from the transparent electrodeside, and the plurality of independent electrodes and their drive system(for example, a thin film transistor) are located on the rear side ofthe display layer. Therefore, a problem of shadow due to the thin filmtransistor or the like is resolved, and high quality displays arerealized.

In the driving method for the display unit according to the invention,extra coloration (discoloration) of the display layer is decreased oreliminated even when coloration occurs since a drive current is appliedall over the display layer having a common potential due to thetransparent electrode or even when a drive current spreads inside of theion conductive layer, since the amount of accumulated electric chargesof the display layer are controlled by the amount of controllingelectric charges or directions of the drive current.

Other and further objects, features and advantages of the invention willappear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an outline cross sectional view showing a configurationexample of a display unit according to a first embodiment of theinvention;

FIG. 2 is a block diagram of the display unit illustrated in FIG. 1;

FIG. 3 is a figure to explain an example of driving method for thedisplay unit illustrated in FIG. 1;

FIG. 4 is a figure to explain other example of driving method for thedisplay unit illustrated in FIG. 1;

FIG. 5 is an exploded perspective view showing in a standard model adisplay condition according to the driving method illustrated in FIG. 4;

FIG. 6 is a view showing a modification of the driving method accordingto the first embodiment of the invention;

FIG. 7 is an exploded perspective view showing in a standard model aconfiguration and a display condition of a display unit according to asecond embodiment of the invention;

FIG. 8 is a view showing an outline configuration and a colorationdensity distribution of a display unit according to a third embodimentof the invention;

FIG. 9 is a view showing an outline configuration and a colorationdensity distribution of a display unit according to a fourth embodimentof the invention; and

FIG. 10 is an outline oblique perspective view showing a configurationof an electrolytic bath used for electrolytic polymerization in anexample of the invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Embodiments of the invention will be described in detail hereinbelowwith reference to the drawings.

[First Embodiment]

FIG. 1 schematically shows a cross sectional structure of a display unitaccording to a first embodiment of the invention. This display unit hasa construction, wherein a display layer 2 and an ion conductive layer 3are arranged between a transparent electrode 1 formed on a transparentsupport 5 and a plurality of picture electrodes 4 (three pictureelectrodes in FIG. 1) formed on a support 7 on the rear side. Thedisplay layer 2 displays characters, images or the like by combiningfine picture elements. The displayed characters, images or the like areviewed from the transparent electrode 1 side through the transparentsupport 5 and the transparent electrode 1. FIG. 1 shows a condition, forexample, wherein a part of the display layer 2 facing a middle pictureelectrode 4 of the three picture electrodes 4 is colored. In addition,this display unit is driven, for example, by an active matrix method,and each picture electrode 4 is electrically connected to a thin filmtransistor (TFT) 6 as a corresponding active device.

The transparent electrode 1 is formed approximately all over thetransparent support 5 as a common electrode. It is preferable to use,for example, a mixture of In₂O₃ and SnO₂, so-called ITO film, or a filmcoated with SnO₂ or In₂O₃ for the transparent electrode 1. It ispossible to use the ITO film and a film coated with SnO₂ or In₂O₃ whichare doped with Sn or Sb, and MgO, ZnO and the like.

As the transparent support 5, a transparent glass substrate such as asilica glass plate and a soda lime glass plate can be used, but itsmaterial is not limited to the above. Other examples for its materialinclude ester such as polyethylene naphthalate and polyethyleneterephthalate; cellulosic ester such as polyamide, polycarbonate andacetylcellulose; fuluoride polymer such as polyvinylidene fluoride andtetrafluoroethylene-hexafluoro propylene copolymer; polyether such aspolyoxymethylene; polyolefin such as polyacetal, polystyrene,polyethylene, polypropylene, and methyl pentene polymer; and polyimidesuch as polyimide-amide and polyetherimide. In the case where thesesynthetic resins are used for a support, the support can be made in theform of a rigid substrate which is hard to be bent, and can be also madein the structure of a flexible film.

The display layer 2 is formed on the transparent electrode 1, andcharacters, images and the like are displayed by discoloring thisdisplay layer 2 corresponding to accumulated electrical charges. Thedisplay layer 2 contains, for example, an electrochromic material whichis colored or discolored by electrochemical oxidation and reduction; ora material which is colored or faded by electrochemical precipitationand elution. For example, in the case that the electrochromic materialwhich is colored or discolored by electrochemical oxidation andreduction is contained, when anion is doped by application of potential,the electrochromic material absorbs more electrons, and dignified blackis displayed.

As the electrochromic material which is colored or discolored byelectrochemical oxidation and reduction, a given material which showsthe electrochromic character, for example, a transition metal compoundsuch as tangstic oxide, iridium oxide and molybdenum oxide; and a rareearth diphthalo cyanine compound such as ruthenium diphthalo cyanine canbe used. However, π conjugated system conductive polymer is suitable,since it can display dignified black.

Examples of π conjugated system conductive polymer includepolyacetylene, poly (p-phenylene), polythiophene, poly (3-methylthiophene), polyisothianaphthene, poly (p-phenylene sulfide), poly(p-phenylene oxide), polyaniline, poly (p-phenylene vinylene), poly(thiophene vinylene), polyperinaphthalene, nickel phthalocyanine and thelike.

One of the most preferable among these π conjugated system conductivepolymers is polypyrrole. The reasons for this include 1) its oxidationpotential is low, 2) its coulomb efficiency is high, 3) color inoxidation is black, 4) its cycle life is long, and the like. The reasonwhy the material having a low oxidation potential is preferred is thatthe material having a low oxidation potential is more stable in colorcondition. The fact that the material having a high coulomb efficiencyis preferred represents that the material having a high coulombefficiency can restrain a side reaction that much, namely, a highcoulomb efficiency of nearly 100% means that a side reaction hardlyoccurs and life as a device becomes longer. The point that color inoxidation is black is an important characteristic as a display fordocuments. Compared to green or reddish black in other polymers, thepolypyrrole provides black color during complete oxidation. Therefore,by adopting the polypyrrole, black density can be raised, and a contrastcan be improved. Further, its long cycle life is one of the beneficialcharacteristics of the polypyrrole.

Though a material which is colored or faded by electrochemicalprecipitation and elution is not limited particularly, examples of thematerial include each ion of bismuth, copper, silver, lithium, iron,chrome, nickel, and cadmium, or combinations of these ions.

The ion conductive layer 3 is formed on the display layer 2 on the sideopposite to the transparent electrode 1. The ion conductive layer 3 isarranged to provide the display layer 2 with ions (anions), and made ofa polymer solid electrolyte wherein a supporting electrolyte isdispersed in a matrix polymer material. Examples of the matrix (hostmaterial) polymer include polyethylene oxide, polyethylene imine, andpolyethylene sulfide, whose respective framework units are shown as—(C—C—O)n-, —(C—C—N)n-, and —(C—C—S)n-. Branched structure having theabove framework unit as a main chain structure is possible. In addition,polymethyl methacrylate, polyvinylidene fluoride, polyvinylidenefluoride chloride, and polycarbonate and the like are also preferable.

When the ion conductive layer 3 is formed, it is preferable to add arequired plasticizer to the matrix polymer. As a preferable plasticizer,water, ethyl alcohol, isopropyl alcohol, and mixtures of them and thelike are preferable when the matrix polymer is hydrophilic; andpropylene carbonate, dimethyl carbonate, ethylene carbonate,γ-butyrolactone, acetonitrile, sulfolane, dimethoxyethane, ethylalcohol, isopropyl alcohol, dimethyl formamide, dimethyl sulfoxide,dimethyl acetamide, n-methyl pyrrolidone, and mixtures of them arepreferable when the matrix polymer is hydrophobic.

As described above, the ion conductive layer 3 is formed by dispersingthe supporting electrolyte in the matrix polymer. Examples of theelectrolyte include, for example, lithium salts such as LiCl, LiBr, LiI,LiBF₄, LiClO₄, LiPF₆ and LiCF₃SO₃; potassium salts such as KCl, KI, andKBr; natrium salts such as NaCl, NaI, and NaBr; and tetraalkyl ammoniumsalts such as tetraethyl ammonium fluoroborate, perchloric acidtetraethyl ammonium, tetrabutyl ammonium fluoroborate, perchloric acidtetrabutyl ammonium, and tetrabutyl ammonium halide. Alkyl chain lengthsof the foregoing tetra ammonium salts can be irregular.

It is possible to add a coloring agent such as a white pigment to theion conductive layer 3 in order to improve the contrast. As the whitepigment, titanium oxide, alumina and the like can be used, andadditionally, zinc oxide and the like can be also used. A mixture ratioof the white pigment is preferably in the range from about 1 to 20 wt %,and more preferably in the range from about 1 to 10 wt %, and mostpreferably in the range from about 5 to 10 wt %. The reason why themixture ratio of the white pigment is limited to such ratios is that thewhite pigment such as titanium oxide is not soluble in polymers and isonly dispersed, so that in the case that its mixture ratio is increased,the white pigment becomes aggregated, resulting in an uneven opticaldensity. In addition, since the white pigment has no ion conductivity,increased mixture ratio causes lowering of conductivity of the polymersolid electrolyte. Considering the both reasons, the upper limit of themixture ratio of the white pigment is about 20 wt %.

The picture electrodes 4 are provided on the ion conductive layer 3 onthe side opposite to the display layer 2, corresponding to pictureelements. The picture electrode 4 is made of a conductive film which isformed in the form of an approximate rectangle or a square pattern. Thepicture electrodes 4 are separated physically and electrically betweeneach other. Each picture electrode 4 is provided with the TFT 6. As amaterial for the picture electrodes 4, a transparent electrode materialcan be used as in the transparent electrode 1. For example, a mixture ofIn₂O₃ and SnO₂, so-called ITO film, or a film coated with SnO₂ or In₂O₃can be used. It is possible to use such ITO film, or a film coated withSnO₂ or In₂O₃ which are doped with Sn or Sb, and MgO or ZnO can be used.

Needless to say, not only the transparent electrode material, but also agiven conductive material such as electrochemically stable metals can beused. Platinum, chrome, aluminum, cobalt, palladium and the like arepreferable. In this case, a film made of a good conductor such as ametal film is formed on the support 7 described later. Further, carboncan be used as a common electrode. As a method to support the carbon onthe electrode, there is a method wherein printing is made on thesubstrate face by using a resin as an ink. By using the carbon, price ofthe electrode can be reduced.

A ratio of a length L of the picture electrode 4, and a distance dbetween the electrodes (a distance between the picture electrode 4 andthe transparent electrode 1) is preferably 3:1 or more. The reason forit will be described later.

The TFT 6 is an active device to perform a switch function for thecorresponding picture electrode 4. The active matrix method wherein thepicture electrode 4 is driven by using the TFT 6 in this way is veryeffective to prevent cross talks between picture elements. The TFT 6 is,for example, formed to occupy one corner of the picture electrode 4 asshown in FIG. 1, but the picture electrode 4 and the TFT 6 can beoverlapped in the direction of the layers. A construction of the TFT 6can be selected as appropriate based on conditions such as a materialfor the support 7 described below.

The picture electrode 4 and the TFT 6 are formed on the support 7 whichis provided on the rear side. The support 7 is not necessarilytransparent, and a substrate or a film which can surely support thepicture electrode 4 and the TFT 6 can be used. For example, a glasssubstrate such as a silica glass plate and a soda lime glass plate, aceramic substrate, a paper substrate, and a wood substrate can be used.However, as a synthetic resin substrate, ester such as polyethylenenaphthalate and polyethylene terephthalate; cellulosic ester such aspolyamide, polycarbonate and acetylcellulose; fuluoride polymer such aspolyvinylidene fluoride and polytetrafluoroethylene-cohexafluoroproplylene; polyether such as polyoximethylene; polyolefin such aspolyacetal, polystyrene, polyethylene, polypropylene, and methyl pentenepolymer; and polyimide such as polyimide-amide and polyetherimide can beused as well. In the case where these synthetic resins are used for asupport, the support can be made in the form of a rigid substrate whichis hard to be bent, and can be also made in the structure of a flexiblefilm.

At a rim part of this display unit, a sealing resin part which supportsthe both supports 5 and 7 is formed (not shown in the figure). The bothsupports 5 and 7, and the transparent electrode 1, the display layer 2,the ion conductive layer 3, the picture electrodes 4, and the TFTs 6which are provided between the supports 5 and 7 are surely supported bythis sealing resin part.

FIG. 2 is a block diagram showing the display unit with theelctrochromic method as shown in FIG. 1. The picture electrodes 4corresponding to each picture element and the TFTs 6 corresponding tothem are arranged in the form of a matrix, and a capacity facingelectrode side is a common electrode. A gate line (a scanning linewiring) 12 is connected to a gate electrode of the TFT 6, and a dataline (signal line wiring) 13 is connected to one side of the source anddrain of the TFT 6. The other side of the source and drain of the TFT 6is connected to the picture electrode 4. The gate lines 12 are connectedto a gate line drive circuit 10, and the date lines 13 are connected toa data line drive circuits 9, 9A. The gate line drive circuit 10 and thedata line drive circuits 9, 9A are connected to a signal controller 11.

Next, a driving method for the display unit according to this embodimentwill be described with reference to FIGS. 3 and 5.

This display unit can be driven by, for example, a line sequentialdrive. Namely, when the gate line drive circuit 10 sequentially appliesselective pulses to the gate line 12 during 1 frame, simultaneously thedata line drive circuits 9, 9A sequentially apply display signalscorresponding to the selected gate line 12 to each data line 13. Throughthe TFT 6 connected to the selected gate line 12, the display signalsapplied to the data line 13 are written from the picture electrode 4side, and characters, images or the like are displayed on the displaylayer 2.

As for writing, by supplying a certain amount of current for a giventime corresponding to the display signals with, for example, so-calledpulse drive, a drive current having electrical charges corresponding toa coloration density (current multiplied by time) can be surely appliedto each picture element of the display layer 2, and a stable contrastcan be obtained.

Here, electrical charges of the drive current are preferably not overtwice as much as the electrical charges at which coloration of a part ofthe display layer 2 which is sandwiched between the picture electrodes 4and the transparent electrode 1 provided with the drive current issaturated. The reason for it is that even when the drive current appliedto one of the picture electrodes 4 flows and makes coloration all overthe display layer 2 which has a common potential due to the transparentelectrode 1, a current distribution is the highest in the area justabove the picture electrode 4 to which the drive current is applied, andmore distant from that part it is, more lower the current distributionis. Therefore, electrical charges flowing into adjacent picture elementsor peripheral picture elements of the display layer 2 can be suppressedto be lower. Consequently, the amount of accumulated electrical chargesof the display layer 2 can be controlled so that extra coloration of thedisplay layer 2 can be reduced, and a major effect on the practicallyadjacent picture elements can be avoided, and therefore quality with noproblems as a display device can be obtained. It is further preferablethat electrical charges of the drive current is suppressed not over theelectrical charges at which coloration of a part of the display layer 2,which is sandwiched between the picture electrodes 4 and the transparentelectrode 1 provided with the drive current is saturated.

FIG. 3 shows in a standard model an example of a coloration densitydistribution of six picture elements arranged along one gate line 13, inthe case where, as described above, drive is performed by limitingelectrical charges to a certain value and under. When the TFT 6 isselectively turned on corresponding to an image, for example, a pulsecurrent is supplied to the second picture electrode 4 from the left andthe second picture electrode 4 from the right, the whole display layer 2is colored since the display layer 2 has a common potential due to thetransparent electrode 1; but the coloration densities in the areas justabove the second picture electrodes 4 from the left and right becomehigher compared to those of others. In the display unit of thisembodiment, since picture elements are not defined by the pictureelectrodes 4, the picture elements may be a little indistinct. However,since one data of image information is given to one picture element,lack of information amount does not occur even when their boundaries areindistinct. It would rather result in good display for photos or thelike, since boundaries between picture elements are not outstanding.

Further, from the view point of a configuration of the display unit, aratio of the length L of the picture electrode 4 and the distance dbetween the electrodes (the distance between the picture electrode 4 andthe transparent electrode 1) is preferably 3:1 or more. The reason forit is, in this manner, a drive current is prevented from spreading inthe ion conductive layer 3, and effect on the adjacent picture elementscan be reduced.

As other method to adjust the amount of accumulated electrical chargesof the display layer 2, a direction of the drive current can beinverted. For example, as shown in FIG. 3, when coloration occurs allover the display layer 2 due to application of the drive current, acurrent whose direction is inverted compared to the direction of thedrive current can be supplied to all the picture electrodes 4 at once,every time writing during one frame is finished. In this manner, asshown in FIG. 4, a certain amount of coloration can be deducted equallyfrom the whole display layer 2, and coloration area returns to theoriginally intended size. Here, in the case where the amount ofelectrical charges of the inverted current are larger than the amount ofelectrical charges of the drive current, displayed images areeliminated, so that electrical charges of the inverted current should beless than the amount of electrical charges of the drive current. Namely,time for applying the inverted current should be set to being very shortcompared to time for supplying the drive current.

FIG. 5 is an exploded perspective view showing in a standard model adisplay condition when a direction of the drive current is inverted forall the picture electrodes 4 as shown in FIG. 4. Patterns of the pictureelectrodes 4 and the TFTs 6 on the base cannot be seen, and onlycharacters are visible on the white background. This is especiallysuitable to display characters which require clearness of outlines.

Though the inverted current can be applied to all the picture electrodes4 at once as shown in FIG. 4, it is also possible to apply the invertedcurrent to the picture electrodes 4 corresponding to outline parts ofdisplay at once. In this manner, extra coloration (discoloration) aroundthe picture elements due to spread of the drive current in the ionconductive layer 3 can be eliminated. Consequently, blur and indistinctcondition of the picture elements can be remedied, and clear displaysbecome possible.

As above, according to this embodiment, since the picture electrodes 4are formed on the ion conductive layer 3 on the side opposite to thedisplay layer 2, characters and images displayed by the display layer 2are viewed from the transparent electrode 1 side, and the pictureelectrodes 4 and the TFTs 6 are located on the rear side of the displaylayer 2. Therefore, optical transmittance of the TFT substrate does notmatter, and the problem of shadow due to the TFTs 6, wiring electrodesof the gate line 12 and the data line 13 and the like is resolved.Further, since patterns of the picture electrodes 4 and the TFTs 6 arenot viewed from the observer side, a real white background is obtained,and a high-quality display can be realized. On the contrary, since in aconventional and ordinary arrangement, an electrochromic display layeris viewed through the TFT side, display becomes dark by a factor of anarea occupied by the TFT, resulting in lowered contrast. According tothis embodiment, differing from the conventional method, since colorchange of the display layer 2 is viewed directly (only through thetransparent electrode 1), there is no parallax or no effects on theoptical transmittance due to the TFT 6, and a bright and high-contrastdisplay can be obtained.

Further, not only an area for the TFTs 6 can be secured maximally anda-Si TFTs and organic TFTs can be utilized, but also the pictureelectrodes 4 are not necessarily made of a transparent material, and agiven electrode material can be used. Furthermore, patterning of thedisplay layer 2 and the transparent electrode 1 is unnecessary, and bigmanufacturing benefits such as reduction of a number of processes can beobtained.

Further, since the amount of accumulated electrical charges of thedisplay layer 2 is controlled by controlling the amount of electricalcharges or directions of the drive current, extra coloration(discoloration) of the display layer 2 is decreased or eliminated evenwhen a drive current is applied and coloration occurs all over thedisplay layer 2 having a common potential due to the transparentelectrode 1 or even when a drive current spreads inside of the ionconductive layer 3. Consequently, a major effect on the practicallyadjacent picture elements can be avoided, and quality with no problemsas a display device can be obtained.

In particular, since electrical charges of the drive current are limitedto not over twice as much as the electrical charges at which colorationof a part of the display layer 2 which is sandwiched between the pictureelectrodes 4 supplied with the drive current and the transparentelectrode 1 is saturated, namely totally reacted, the electrical chargeswhich flow in the adjacent or peripheral picture elements to the displaylayer 2 can be suppressed. In result, major effect on the practicallyadjacent picture elements can be avoided, and it would rather result ingood display for e.g. photos since boundaries between picture elementsare not outstanding.

Further, specifically, since a ratio of the length L of the pictureelectrode 4 and the distance d between the electrodes (the distancebetween the picture electrode 4 and the transparent electrode 1) is setto 3:1 or more, spread of the drive current in the ion conductive layer3 is suppressed, and effects on the adjacent picture elements can bereduced.

Further, since a direction of the drive current is inverted, theelectrical charges leaked to the peripheral picture elements are sweptout, so that characters, images and the like can be satisfactorydisplayed on the display layer 2, and a bright and parallax-freereflective display can be realized.

Specifically, since the inverted current is supplied to all the pictureelectrodes 4 at once, a certain amount of coloration can be deductedequally from the whole display layer 2, and a coloration area returns tothe originally intended size. Therefore, patterns of the pictureelectrodes 4 and the TFTs 6 on the base are not seen, and onlycharacters are visible on the white background. This is especiallysuitable to display characters requiring clearness of outlines.

[Modification]

FIG. 6 is a view showing a modification of the driving method for theforegoing first embodiment. It is needless to say that gradation displayto change coloration density of each picture element is possible bymodifying the current supply time in the first embodiment. In thismodification, the gradation display is made by modifying a colorationarea of each picture element of the display layer 2, in other words, byusing so-called area gradation. In the case that the display layer isformed on the picture electrode as conventional, a coloration area isdetermined by the electrode area of the picture electrode. However,according to this modification, by using coloration spread due to spreadof the drive current in the ion conductive layer 3, the area gradationdisplay becomes possible.

In this modification, it is preferable that the length L of the pictureelectrode 4 is shorter than in the first embodiment, in order toactively use the spread of the drive current and achieve a moderatelyclear display.

[Second Embodiment]

FIG. 7 is an exploded perspective view showing in a standard model aconfiguration and a display condition of a display unit according to asecond embodiment of the invention. The display unit of this embodimentis driven by a simple matrix method, and comprises a picture electrode24 which is formed as a group of strip-shaped electrodes parallel toeach other, and a transparent electrode 21 which is formed as a group ofstrip-shaped transparent electrodes parallel to each other, which isperpendicular to the picture electrode 24. At intersections of thepicture electrode 24 and the transparent electrode 21, picture elementsare arranged. Respective materials for the transparent electrode 21 andthe picture electrode 24 are the same as those for the transparentelectrode 1 and the picture electrode 4 in the foregoing firstembodiment. Since components other than the transparent electrode 21 andthe picture electrode 24 are the same as those in the foregoing firstembodiment, the same symbols are applied to the same components anddescriptions for them are omitted. The transparent electrode 21 isformed on the transparent support 5 (not shown in FIG. 7. Refer to FIG.1), and the transparent support 5 is arranged on the transparentelectrode 21 on the side opposite to the display layer 2. The pictureelectrode 24 is formed on the support 7 (not shown in FIG. 7. Refer toFIG. 1), and the support 7 is arranged on the picture electrode 24 onthe side opposite to the display layer 2.

In this display unit, in sync with supplying a scanning signal having apulse width corresponding to a scanning selection period to thetransparent electrode 21, a display signal having a pulse widthcorresponding to a coloration density is supplied to the pictureelectrode 24. In the case of such simple matrix method, for example,after writing for every line of the transparent electrode 21, adirection of the drive current is inverted and the current invertedopposite to the direction of the drive current is applied to the wholelines in the same manner as described in the first embodiment withreference to FIG. 4. In this way, outlines become clear, and a displayof good images and characters becomes possible. Application of theinverted current can be performed for every line, or for the all linesof the transparent electrode 21 after writing for one frame is finished.In view of a cross section along a given line of the transparentelectrode 21 (single dotted line of FIG. 7), its operation principle issimilar to that of the active matrix method.

According to this embodiment, since the simple matrix drive, which hasbeen conventionally difficult as a matter of fact due to cross talk, canbe adopted in the current drive type electrochromic display unit, anelectrochromic display unit with further lower cost and excellent imagequality can be provided.

[Third Embodiment]

FIG. 8 is a view showing an outline configuration of a display unitaccording to a third embodiment of the invention, and showing an exampleof a coloration density distribution of six picture elements arrangedalong one gate line 13 in a standard model. This display unit has thesame configuration as in the display unit of the first embodiment shownin FIG. 1 except that grooves 3C are provided on the ion conductivelayer 3 made of a solid electrolyte by selectively eliminating areasbetween picture elements. Therefore, the same symbols are applied to thesame components, and detailed descriptions for them are omitted. In thisdisplay unit, since the active matrix drive using the TFT 6 is adoptedas in the display unit of the first embodiment, and its driving methodis the same as the driving method described based on FIGS. 3 and 4, itsdetailed description is omitted.

The grooves 3C provided on the ion conductive layer 3 are, for example,void parts, and the ion conductive layer 3 is divided for every pictureelement by the grooves 3C. Therefore, for example, in the case where adrive current is only applied to the second picture electrodes 4 fromthe left and the right so that they have the same electrical charges asin FIG. 3, the drive current focuses on the areas above the pictureelectrodes 4 as shown in a coloration density distribution graph of FIG.8 and a clear display becomes possible.

As a concrete technique to selectively eliminate the ion conductivelayer 3 to form the grooves 3C and divide the ion conductive layer 3spatially, sand blast method, laser process method and the like can belisted. The sand blast method is the technique to partly shave a solidmaterial by rapidly spraying gas such as air containing fine powderssuch as silicon dioxide from a fine nozzle. When the picture electrodes4, or a mask material to form the picture electrodes 4 and a thicknessof the mask material is appropriately selected, self-aligning shaving ofthe ion conductive layer 3 between the picture elements is possible byusing the above as a mask.

The laser process is the technique wherein high polymer is locallyevaporated by using, for example, a strong ultraviolet laser to form avoid. As in the foregoing sand blast method, when the picture electrodes4, or a mask material to form the picture electrodes 4 and a thicknessof the mask material is appropriately selected, self-aligning laserprocess can be performed by using the above as a mask against theultraviolet laser. It is also possible to form a sequential long grooveby forming a line-shaped beam by using a light transmittance type maskand a cylindrical lens on the slit, and by scanning the beam inparallel. Examples of such laser for microfabrication include anultraviolet pulse laser. Though a carbon dioxide gas laser and a YAGfundamental wave infrared laser can be used, with such laser, it isdifficult to perform microfabrication to about 50 μm or under sincematerials are scattered due to dissolution and boiling. By using theultraviolet laser, chemical bonding can be directly cut, andhigh-precision process with a little residue is possible.

Concrete examples of the laser used are as follows:

1) Excimer Laser

-   -   (Pulse width: 10 to several 10 ns, cycle frequency: to 200 Hz)        XeF: 351 nm, XeCl: 308 nm (for silicone anneal), KrF: 248 nm,        ArF: 193 nm.

2) Q Switch YAG Laser

-   -   (Pulse width: several ns, cycle frequency: to 10 Hz/lamp        excitation; to 10 kHz/LD excitation)    -   Threefold wave: 355 nm, fourfold wave: 266 nm

Process energy density of 500 mJ/cm² (per pulse) or more is required,and shaving of about 0.1 to 1 μm per 1 pulse is available. Availableshaving depth is determined by an absorption coefficient and a power.Among the foregoing lasers, the KrF laser is suitable in terms ofefficiency, output, and stability. A process width is about 5 μm. When afurther microfabrication, a process for the one whose wavelength has ashort absorption edge (inorganic oxide and the like), clean process (inthe case of organic matter, the longer the wavelength is, the more soildue to carbon or the like is applied) and the like are necessary, it ispreferable to use the ArF laser, the YAG fourfold wave and the like.

In order to prevent the adhesion of the carbon, it is also effective toradiate the laser while spraying oxygen gas. In order to process in theanaerobic atmosphere and prevent re-adhesion of the scattered objects,it is also possible to radiate the laser in the vacuum or He gasatmosphere.

According to this embodiment, since the ion conductive layer 3 isdivided for every picture element by the grooves 3C, the void partsprovided in the areas between the picture elements, a drive current doesnot spread in the ion conductive layer 3 and the drive current focuseson areas 3A above the picture electrodes 4, so that a clear displaybecomes possible. In addition, since there is no need to consider spreadof the drive current in the ion conductive layer 3, the distance dbetween the electrodes (the distance between the picture electrode 4 andthe transparent electrode 1) can be further shortened, and it can beexpected that a thickness of the display unit will be reduced.

[Fourth Embodiment]

FIG. 9 is a view showing an outline configuration of a display unitaccording to a fourth embodiment of the invention, and showing anexample of a coloration density distribution of six picture elementsarranged along one gate line 13 in a standard model. In the thirdembodiment, the ion conductive layer 3 is divided spatially by selectiveelimination. In this embodiment, the ion conductive layer 3 is dividedfor every picture element by selectively lowering or selectively raisingion conductivity of the ion conductive layer 3. Namely, the display unitof this embodiment has the same configuration as in the display unit ofthe first embodiment shown in FIG. 1 except that the ion conductivelayer 3 is composed so that an ion conductivity of the areas 3Acorresponding to the picture elements is higher than an ion conductivityof areas 3B between the picture elements. Therefore, the same symbolsare applied to the same components, and detailed descriptions for themare omitted. Further, in this display unit, the active matrix driveusing the TFT 6 is adopted as in the display unit of the firstembodiment, and its driving method is the same as in the descriptionbased on FIGS. 3 and 4, so that its detailed descriptions are omitted.

Specifically, for example, in the case where a conductive polymer isformed by polymerisation in preparation of a solid electrolyte, theresistance in that area can be raised or lowered by partly causingchemical change such as cross-link and the like by ultraviolet. In thecase of using lights, when the picture electrodes 4, or a mask materialused to form the electrodes 4 and its thickness is appropriatelyselected, self-aligning method by using them as a mask can be applied,and big advantages are obtained in terms of manufacturing cost andyield.

According to this embodiment, by lowering the ion conductivity in otherwords, by raising the resisitivity, of the areas 3B between the pictureelements than the ion conductivity of the areas corresponding to thepicture elements, namely, of the areas 3A located above the pictureelectrodes 4, spread of the drive current in the ion conductive layer 3can be suppressed, and blur of the picture elements can be excluded.

Concrete examples of the invention will be described below based onexperimental results. It goes without saying that the invention,however, is not to be limited to such examples.

EXAMPLE 1

(Production of a Display Pole)

After forming an ITO film uniformly over a glass substrate with athickness of 1.1 mm and dimensions of 10 cm×10 cm, a lead part wasformed at the edge part of the substrate by a known method. As shown inFIG. 10, this glass substrate 31 was located in a glass bath forelectrolytic polymerization 32. An electrolytic solution in the glassbath 32 was obtained by dissolving tetraethyl ammonium tetrafluoroborate of 1 mol/l and pyrrole of 1 mol/l in propylene carbonate. Inaddition to the glass substrate 31, a platinum substrate 33 as a counterelectrode and a silver wire 34 as a reference electrode were arranged inthe glass bath for electrolytic polymerization 32 as shown in FIG. 10.

Subsequently, from an unshown drive circuit, a constant current of 2 mAwas entirely applied until a current-carrying quantity became 3C (30mC/cm²). On the ITO, an electrolytic polymerisation film of polypyrroleshowing black color caused by doping of tetrafluoro borate anion wasformed. Then, the glass substrate 31 was arranged in a glass bathcontaining an electrolytic solution obtained by dissolving tetraethylammonium tetrafluoro borate of 1 mol/l in propylene carbonate, a currentof −1 mA was applied until a current-carrying quantity became 0.8 C (8mC/cm²), and ions doped in polypyrrole during polimerisation werededoped. Color of the electrolytic polymerisation film of polypyrrolechanged to slightly yellowish transparent color.

(Preparation and Application of a Polymer Solid Electrolyte)

8 parts by weight from polyvinylidene fluoride with molecular weight ofabout 0.35 million and tetraethyl ammonium tetrafluoro borate of 1 molwere dissolved in propylene carbonate. Subsequently, titanium oxide of25 parts by weight with its average particle diameter of 0.1 μm wasadded to the above mixture, and the resultant was uniformly dispersed byan ultrasonic homogenizer. The foregoing substrate was spin coated withthis polymer solution under conditions of 1,000 rpm for 4 sec, and then3,000 rpm for 30 sec. The resultant was dried under reduced pressure at110° C. in 0.1 Mpa for 1 hour. Immediately after gelation, the resultantwas affixed to a drive pole described later, and a polymer solidelectrolyte was formed between two electrodes as an ion conductivelayer. Then, the edge of the affixed part was sealed by using an epoxyultraviolet curing resin (Photolec made by Sekisui Chemical Co., Ltd.)as a sealant.

(Production of a Drive Pole)

On a glass substrate with a thickness of 1.1 mm and dimensions of 10cm×10 cm, an ITO film and TFTs arranged two-dimensionally in 150 μmpitch were produced by a known method. Subsequently, a lead partconnecting to the drive circuit was produced on the glass substrate by aknown method.

(Evaluation of the Drive and Display Characteristics)

Black display and colorless (white) display were switched by oxidizingthe display pole with an electrical quantity of 2 μC per 1 pictureelement during coloring and reducing the display pole with the sameelectrical quantity during discoloring by the known active matrix drivecircuit.

Reflectance in colorless (white) condition was 70%, and optical density(OD) of the display part in coloring (black) condition was about 1.3(reflectance of 5%). Therefore, as a contrast of the reflectances, 1:14was obtained.

On the 7th day after being kept in coloring condition and left with itscircuit open, the optical density of the display part was about 1.0, andmemory characteristic existed. In the case where cycles of coloring anddiscoloring were repeated, the cycle number of times until black densityduring coloring became 1.0 or less was about 8 million.

EXAMPLE 2

In order to examine how much blur occurs outside of the pictureelements, polypyrrole was deposited on a line electrode (width of 4 mm)as a first electrode and on an allover electrode as a second electrodeunder the same conditions as in the Example 1. An electrochromic displaylayer was made of polypyrrole (polymerisation conditions: constantcurrent of 2 mA, synthetic electrical quantity of 30 mC/cm²), anelectrolyte was made of tetraethyl ammonium tetrafluoro borate, and thethickness of an ion conductive layer was 200 μm.

Based on a response speed of the part where the electrodes are formed onthe both substrates (polypyrrole was also formed on the bothelectrodes), a response speed of the part at a distance of 1.1 mm fromthe electrodes was compared. The response speeds were measured under atransmission microscope, and detected with photomaltiplier strength. Thedrive wave form was rectangle of 0.1 Hz, and the impressed voltage was±1 V. Consequently, the response speed of the polypyrrole was 190 ms atthe part where there were the electrodes on the both substrates, and theresponse speed of the polypyrrole at the part where there was theelectrode only on one substrate, at a distance of 1.1 mm from the edgeswhere there were the electrodes on the both substrate was longer thanthe above speed by 160 ms.

While the invention has been described with reference to the embodimentsand the examples, the invention is not limited to the foregoingembodiments and examples, and various modifications may be made. Forexample, in the first embodiment, the method to limit the electricalcharges of the drive current to a certain value or less as shown in FIG.3 and the method to invert the direction of the drive current as shownin FIG. 4 were described. However these methods can be used at the sametime, and without mentioning, only one of them can be used.

Further, for example, in the third embodiment, the grooves 3C were voidparts. However, it is possible to fill an insulation material in thegrooves 3C.

In addition, for example, in the third and the fourth embodiments, thecases of the active matrix drive by the TFT were described as examples.However, the configuration wherein the ion conductive layer 3 is dividedfor every picture element as in the third and the fourth embodiments canalso be adopted for the case of the simple matrix drive.

As described above, according to the display unit of the invention,since a plurality of independent electrodes are formed on the ionconductive layer on the side opposite to the display layer, charactersand images displayed by the display layer are viewed from thetransparent electrode side, and the plurality of electrodes and the TFTsconnected to the plurality of electrodes as active devices and the likeare located on the rear side of the display layer. Therefore, opticaltransmittance of the TFT substrate becomes unconsidered and the problemof shadow due to the TFT, a wiring electrode or the like is solved. Inaddition, since patterns of the plurality of electrodes and the TFTs arenot viewed from the observer side, the display layer becomes a realwhite background, and a high-quality display can be realized. On thecontrary, in the conventional and general arrangement, since theelectrochromic display layer is viewed through the TFT side, the displaybecomes dark by a factor of the area occupied by the TFT, resulting inlowered contrast. According to the invention, different from theconventional method, color change of the display layer is directly (onlythrough the transparent electrode) viewed, so that there is no parallaxor no effects on optical transmittance due to the TFT, and a bright andhigh-contrast display can be obtained.

Further, not only areas for the TFTs can be secured maximally and a-SiTFTs and organic TFTs can be utilized, but also the plurality ofelectrodes are not necessarily made of a transparent material, and agiven electrode material can be used. Furthermore, patterning of thedisplay layer and the transparent electrode is unnecessary, and bigmanufacturing benefits such as reduction of a number of processes can beobtained.

Particularly, according to the display unit of one aspect of theinvention, since the ion conductive layer is divided for every pictureelement by the groove parts provided in the areas between the pictureelements, the drive current does not spread in the ion conductive layer,and focuses on the areas located above each of the plurality ofelectrodes, resulting in a clear display.

Further, according to the display unit of another aspect of theinvention, the ion conductive layer is composed so that the ionconductivity in the area corresponding to the picture element is higherthan the ion conductivity in the area between picture elements.Therefore, spread of the drive current is suppressed, and blur of thepicture elements becomes unconsidered. Consequently, the display layeris colored only corresponding to the electrodes supplied with the drivecurrent, and a clear display becomes possible.

Further, according to the display of still another aspect of theinvention, the plurality of electrodes is a group of strip-shapedelectrodes parallel to each other, the transparent electrode is a groupof strip-shaped transparent electrodes parallel to each other which isperpendicular to the above plurality of electrodes, and the pictureelements are arranged at intersections of the strip-shaped electrodesand the strip-shaped transparent electrodes. Therefore, the simplematrix drive, which has been practically difficult to use due to crosstalk, can be adopted. Consequently, a display unit with further lowercost and excellent image quality can be provided.

In addition, according to the display unit of still another aspect ofthe invention, in the case of the active matrix drive, the ratio of thelength of the plurality of electrodes and the distance between thetransparent electrode and the plurality of electrodes is set to 3:1 ormore, and in the case of the simple matrix drive, the ratio of the widthof the strip-shaped electrode comprising the plurality of electrodes andthe distance between the transparent electrode and the plurality ofelectrodes is set to 3:1 or more. Therefore, spread of the drive currentin the ion conductive layer is suppressed, and effects on the adjacentpicture elements can be reduced.

According to the driving method for the display unit of the invention,since the accumulated electrical charges of the display layer iscontrolled by controlling electrical charges or directions of the drivecurrent, extra coloration (discoloration) of the display layer isdecreased or eliminated even when coloration occurs since a drivecurrent is applied all over the display layer having a common potentialdue to the transparent electrode or even when a drive current spreadsinside of the ion conductive layer. Consequently, a major effect on theadjacent picture elements can be avoided practically, and quality withno problems as a display device can be obtained.

Specifically, according to the driving method for the display unit ofone aspect of the invention, since electrical charges of the drivecurrent is limited to not over twice as much as the electrical chargeswherein discoloration or coloration of the part of the display layerwhich is sandwiched between the plurality of electrodes and thetransparent electrode provided with the drive current is saturated,namely totally reacted, the electrical charges which flow in theadjacent or peripheral picture elements to the display layer can besuppressed. In result, major effect on the adjacent picture elements canbe avoided practically, and it would rather result in good display fore.g. photos since boundaries between picture elements are notoutstanding.

Further, according to the driving method for the display unit of anotheraspect of the invention, since a direction of the drive current isinverted, characters, images and the like can be well displayed on thedisplay layer, and a bright and parallax-free reflective display isrealized.

Further, according to the driving method for the display unit of stillanother aspect of the invention, since the inverted current is suppliedto all the plurality of electrodes at once, a certain amount ofcoloration can be deducted uniformly from whole the display unit, and asize of coloration area returns to the originally intended size.Therefore, only the areas corresponding to the electrodes provided withthe drive current are colored, patterns of the plurality of electrodesand the TFTs on the base are not visible, and only characters areviewable on the white background. It is especially suitable for the caseto display characters which requires clearness of outlines.

In addition, according to the driving method for the display unit ofstill another aspect of the invention, since the current whose directionis inverted is simultaneously supplied to the electrodes among theplurality of electrodes, corresponding to the outline parts of thedisplay, extra coloration (discoloration) around the picture elementsdue to spread of the drive current in the ion conductive layer can beeliminated. Consequently, blur or indistinct condition of the pictureelements can be remedied, and a clear display becomes possible.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

1. A display unit comprising: a transparent electrode; a display layerformed on the transparent electrode which discolors corresponding toaccumulated electrical charges; and an ion conductive layer formed onthe display layer on the side opposite to the transparent electrode,wherein: a plurality of independent electrodes are formed on the ionconductive layer on the side opposite to the display layer, theplurality of independent electrodes are arranged corresponding topicture elements, and connected to respectively corresponding thin filmtransistors, the transparent electrode is a common electrode, and aratio of a length of each independent electrode and a distance betweenthe transparent electrode and the plurality of independent electrodes is3:1 or more.
 2. A display unit comprising: a transparent electrode; adisplay layer formed on the transparent electrode which discolorscorresponding to accumulated electrical charges; and an ion conductivelayer formed on the display layer on the side opposite to thetransparent electrode, wherein: a plurality of independent electrodesare formed on the ion conductive layer on the side opposite to thedisplay layer, the plurality of independent electrodes are a group ofstrip-shaped electrodes parallel to each other, the transparentelectrode is a group of strip-shaped transparent electrodes parallel toeach other, which is perpendicular to the plurality of independentelectrodes, picture elements are arranged at intersections of thestrip-shaped independent electrodes and the strip-shaped transparentelectrodes, and a ratio of a width of each independent electrode and adistance between the transparent electrode and the plurality ofindependent electrodes is 3:1 or more.